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Unclassified/Distribution A Approved for Public Release 2010 NDIA GROUND VEHICLE SYSTEMS ENGINEERING AND TECHNOLOGY SYMPOSIUM MODELING & SIMULATION, TESTING AND VALIDATION (MSTV) MINI-SYMPOSIUM AUGUST 17-19 DEARBORN, MICHIGAN SURVIVABILITY ENHANCED RUN-FLAT VARIABLE FOOTPRINT TIRES James Capouellez Dr. Abraham Pannikottu US Army RDE-COM American Engineering Group TARDEC, AMSRD-TAR-D, MS 233 934 Grant St, Suite #101, Warren, MI 48397-5000 Akron, OH 44311, USA Dr. Jon Gerhardt American Engineering Group 934 Grant St, Suite #101, Akron, OH 44311, USA ABSTRACT The military has a unique requirement to operate in different terrains throughout the world. The ability to travel in as much varying terrain as possible provides the military greater tactical options. This requirement/need is for the tire to provide a variable footprint to allow for different ground pressure. Much of the current run-flat technology utilized by the military severely limits mobility and adds significant weight to the unsprung mass. This technology gap has allowed for the development of new run-flat tire technology. New tire technology (fig 1) has been developed that substantially increases survivability, eliminates the need for heavy run-flat inserts, significantly reduces air pressure requirements and provides full (or near full) speed capability in degraded/damaged mode (punctured tire). This run-flat technology is built directly into the tire, yet maintains the normal variable footprint of a normal pneumatic tire. This makes the tire/wheel assembly much lighter and far more survivable than normal military run-flat technology. Safety, logistics, economics, and fuel economy are additional benefits this tire technology provides over current military tires with run-flat inserts. INTRODUCTION The current state of the art tire technology utilizes a run-flat insert sometimes weighing over 100lbs. Fig 2 shows a cut-away showing the run-flat for a military tire/wheel. The stability of the vehicle running on a run-flat is significantly reduced and the operator is limited to a maximum speed of 30 mph, and sometimes less depending on stability. The distance that a vehicle can travel with a run-flat tire is also limited to typically 30 miles. This is also very similar to the NATO standard of 50 km [1]. The purpose of the run-flat is to allow the vehicle to have limp home capability and that’s it. Logistically the run-flat has further drawbacks. Unless you buy the wheel, run-flat, and tire as a package, you have to get the run-flat into the tire. This requires Unclassified/Distribution A Approved for Public Release Unclassified/Distribution A Approved for Public Release a special tool to compress profile encountered for different missions and the run-flat and get it into theaters, plus run-flat capability. New military the tire [2]. One such tool wheeled vehicles systems such as the Stryker or M- used on a HMMWV tire is ATV have on-board CTIS systems to enable these shown in Figure 3 [2]. The vehicles to quickly transition from highway mode to larger the run-flat the more off-road, or vice versa, within the safety of the difficult this becomes. This vehicle [3]. Most of the commercial market needs process further adds to the only one tire pressure setting for highway usage. maintenance Automobiles benefit the most from proper tire Figure 2. Military Run-flat burden on the inflation pressure, resulting in premium fuel Tire with Insert [1] military that has economy, extended tire life and proper handling. limited resources. This has lead to the development of stiff sidewall tires that provide similar handling and perform with Although run flat technology has been around for a or without air. However, the tire reliability and while, very little improvement has been made over durability are significantly worse without air the past 25-30 years. The current requirement of the pressure. Consumers of these tires have actually run-flat tire for the HMMWV is the same as it has complained that when the tire lost air pressure they ever been. had no degradation of performance so they kept running the tires until they were damaged beyond repair. This problem is the reason why some cars have tire pressure monitors to notify the driver of a flat tire. This is a far different experience of riding on a tire with a run-flat insert. The stiff sidewall tire requires a very low aspect ratio so the tire can be supported properly by the sidewalls. “The sidewalls can't be very tall, so most Figure 3. HMMWV Run-flat are low-profile designs. Because of this, they are Insert Special Tool and typically used on sports, though they're also available Installation [2] for regular passenger cars and even minivans.”[4] The drawback with this tire is that the stiff sidewalls To make a run-flat insert better than what is tend to put more energy into the vehicles suspension currently produced is extremely difficult. The system and ultimately the vehicle. For new car commercial application for run-flat inserts is limited designs, this can be designed up front. However, the because of cost and performance. The systems military has vehicles that are in inventory for typically costs a lot more because of the additional extensive periods of time and cost to swap out hardware of the insert and the wheel needs to be suspension systems along with the tires can be a cost specially designed for the run-flat insert. Still, the that is hard to justify. Since the runflat is built into a premise of the run-flat insert is running on a small stiff sidewall, how much can the footprint can solid tire. The insert can only be so large or it will changed to provide different ground pressure for act as a bump stop for the tire and the width of the different terrain may be an issue that prevents this run-flat is going to be significantly less than the technology from being used for the military. width of the tire. This limits the surface area that the load can be distributed over. Also, the flat tire must either flop along or degrade due to the heat buildup of running directly on the tire and run-flat insert. This could lead to stability issues, smoke and possible flammability issues. There is a way to sum up the difficulty in making the run-flat performance significantly better, physics. Aside from physics, another reason the development will be slow for the run-flat insert is that the commercial market has different requirements. Figure 4. Comparison of Conventional Tire to a Most military vehicles need to have the capacity to Stiff Sidewall Tire [4] change the tire pressure for the different terrain Unclassified/Distribution A Approved for Public Release Unclassified/Distribution A Approved for Public Release To improve the runflat capability of the United weight of the springs could be reduced making the States military legacy systems, technology needs to concept more desirable. be developed that provides a variable footprint, can operate at zero air pressure with the carcass severely damaged/punctured, provide the same dynamic deflection as the normal pneumatic tire, and provide similar tread life. Could a tire be developed that would provide the same ground pressure requirements as a normal pneumatic tire such that it could meet the military demands of operating in sandy soft soil with zero or minimum air pressure, add air pressure for proper highway speeds and handling, and yet be able to provide enhanced run- flat capability. Could all this capability be developed in the tire to reduce the logistical burden associated with the run-flat inserts? From modeling, simulation, FEA, and laboratory testing the answer appears to be yes. Figure 5. Load and Boundary Conditions Applied on the Model for a First Order Foot Print Analysis Tire Development Further prototype development required integrating How does the pneumatic tire work? The load for the run-flat support into the tire. This would require the axle is resisted by the road. The body plies of the the development of rings or cylinders imbedded in tire between the axle and the road have reduced the tire. The shape of the ring could be optimized tension (resulting in the tire bulge). The cords above utilizing CAD and FEA analysis. the axle are in higher tension and as a result pull up The balance for the ring is going to have to be the axle. The means of putting the cords in tension achieving proper bonding between the ring and the is through air pressure. What other means could be tire rubber yet having the stiffness to support a pre- utilized to put the cords in tension. determined load with a desired change in the vertical diameter to produce the desired ground pressure. Stiff Sidewalls Formulas and analysis for circular rings is much Spring Molded Tire more straight forward. Spring Inserted Tire Assuming no deformation, the change in height is Hoop Ring Molded Tire (Metal) as follows: Hoop Ring Molded Tire (Fiber Glass Rods) (1) WR 3 Carbon Fiber Molded Tire (Layer) Dh 0 . 1488 [5] Hoop Ring Molded Tire (Carbon Fiber) EI To simplify development and test the concept, The ring can fail by either excessive compressive spiral springs were developed and inserted into the forces or by buckling. The critical stress in the ring tire. This was done on a small automotive tire to for buckling can be estimated by replacing the ring as quickly and economically evaluate the concept and a cylinder and doing the stress equation for a hollow help correlate stress analysis conducted on CAD cylinder. The basic equation for column buckling is: models and actual lab testing.